我的想法是，为了只在上游推送了一些提交时才自动获取更新，我应该检查它的散列。
2种方法，我可以做的是，检查logs或使用rev-list的东西如下：

# this fetches top 30 commits from forked repo
git rev-list -n 30 main

# this also does the same but not limited to 30
git log --format='%H'

我发现很难比较哈希值，因为我无法从分支回购中获取超过1个提交。
下面解释了我的方法：

git remote add upstream https://github.com/upstream/upstream.git
git fetch upstream --tags

upstream_commit="$(git rev-parse upstream/main)"
echo "Upstream commit: ${upstream_commit}"

# This should store 30 recent commit hashes as an array AFAIK
forked_commits=($(git rev-list -n 30 main))
# another I tried to put forloop is this:
# for forked_commit in "$forked_commits[@]"; do
for forked_commit in $(git log --format='%H'); do
  # but prints only the top most element and exits the loop
  echo "$forked_commit"
  if [ "$upstream_commit" == "$forked_commit" ]; then
    has_new_commits=false
  else
    # since I've added my commits on top, if condition fails and this always returns true
    has_new_commits=true
  fi
done
          
if [ "${has_new_commits}" ]; then
  git checkout main
  git rebase upstream/main
  git push -f origin main
  echo "Rebase successful!"
else
  echo "No commits to be synced!"
fi